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Full text of "Spectrophotometric determination of praseodymium, neodymium, and samarium"

U. S. Department of Commerce National Bureau of Standards 

RESEARCH PAPER RP1395 

Part of Journal of Research of the Rational Bureau of Standards, Volume 26, 

June 1941 

SPECTROPHOTOMETRIC DETERMINATION OF 
PRASEODYMIUM, NEODYMIUM, AND SAMARIUM 

By Clement J. Rodden 



ABSTRACT 

Praseodymium, neodymium, and samarium, in nitrate solutions, have been 
determined by measuring the transmittancies of the solutions with a double- 
monochromator photoelectric spectrophotometer. 

Spectral transmittancy curves from 350 to 1,000 m/z were obtained for lan- 
thanum, cerium, praseodymium, neodymium, samarium, europium, and gado- 
linium. The absorption band found to be most suitable for the determination of 
praseodymium was at 446 m/x; for neodymium, the bands at 521 and 798 m/*; 
and for samarium, 402 rn.fi. Lanthanum, cerium, and gadolinium show negligible 
absorption between 400 and 1,000 m/*. Europium shows a small absorption band 
at 396 m/i- 

Transmittancy-concentration curves were obtained for the individual elements 
in concentrations ranging from 0.25 to 25 mg/ml. These curves show that for 
concentrations up to 10 mg/ml, solutions of neodymium follow Beer's law, while 
in concentrations greater than 10 mg/ml this law does not hold. Praseodymium 
and samarium solutions do not follow Beer's law under the conditions used. 

Approximately 1 mg of praseodymium per 6 ml can be detected. The sensitivity 
of the neodymium test depends on the band used; 1.5 mg can be detected by using 
the 521 m/x band and 0.5 mg by using the 798 m/i band. The test for samarium 
is less sensitive than for the other two elements, 3 mg being the minimum detected. 

Mixtures of the three elements were analyzed and also mixtures of each of the 
three elements with lanthanum. The mixtures varied from one containing 5.0 
mg of neodymium and 200 mg of lanthanum, to one consisting of 75 mg of sama- 
rium, 75 mg of praseodymium, and 50 mg of neodymium. A weight of mixed 
oxides corresponding to approximately 200 mg of rare earth elements was used 
for each analysis. The accuracy obtained was of the order of ±3 mg. 

A method is given for correcting the slight interference of each element in a 
mixture, and a procedure is outlined for the analysis of a mixture of the cerium 
group of elements obtained during the course of a mineral analysis. 



CONTENTS 

Page 

I . Introduction 557 

II. Experimental 559 

1 . Apparatus 559 

2. Materials 560 

3. Transmittancy measurements 561 

III. Results obtained 567 

IV. Discussion 568 

I. INTRODUCTION 

The term "rare earth metals" is used to designate the elements 
cerium, praseodymium, neodymium, 61, samarium, europium, gado- 
linium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, 

557 



558 Journal oj Research of the National Bureau oj Standards [Volm 

and lutecium, which have the atomic numbers 58 to 71, and which 
are usually grouped with lanthanum in the periodic table. In most 
analytical work, however, scandium (at. No. 21), yttrium (at. No. 
39), lanthanum (at. No. 57), and sometimes thorium (at. No. 90) are 
included with the rare earth metals, because they resemble the rare 
earths in many of their properties. 

Because the rare earths are very similar in their chemical and their 
physical properties, the means of separating them are very limited. 
The analyst can separate scandium, cerium, and thorium from the 
others in the group. He can make use of the relative insolubility of 
the double alkali sulfates of lanthanum, cerium, praseodymium, 
neodymium, samarium, europium, and gadolinium in a saturated 
solution of potassium or sodium sulfate to separate these elements 
(the cerium group) from the remainder (the yttrium group). 1 In 
this way it is possible to obtain elements near the extreme ends of the 
solubility series free from one another, but the division between the 
groups is not sharp. 

The salts of most of the rare earth metals are colored and their 
solutions show characteristic sharp absorption bands in the visible 
and ultraviolet region of the spectrum. 2 The possibility of using these 
absorption bands as a means of determining the respective elements 
in mixtures has been recognized for a considerable time. Four differ- 
ent methods have been used for the estimation of the rare earths by 
means of their absorption spectra. In the oldest method, which is 
based on the Nessler principle, the test solution is diluted until, with a 
direct-vision spectroscope, the absorption intensities of suitable 
bands appear to equal those of a standard solution placed in juxta- 
position. 3 This method has been used chiefly to determine impurities 
in so-called pure salts of the rare earths. Friend and Hall 4 used a 
modification of this method and stated that, " Whilst it is apparently 
impossible to determine directly with an approach to accuracy the 
amount of praseodymium nitrate in the presence of neodymium 
nitrate, the latter can readily be estimated in presence of up to 50% 
of the former in 4% solution." 

The second method is based on finding the dilution necessary for 
the disappearance of characteristic absorption bands. Haas 6 de- 
termined the concentration of neodymium and of praseodymium in 
mixtures from the absorption spectra of the nitrates, by diluting the 
solutions until the neodymium band at 521 m/x (green region) could 
no longer be seen. The band at 480 m/i (violet region) was used for 
the determination of praseodymium. It was found that lanthanum 
did not interfere. Schumacher 6 determined praseodymium and 
neodymium in a samarium preparation by the same method. Yn- 
tema 7 suggested photographic recording in a similar method. 

The third method is that of Delauney, 8 who plotted curves to show 
the relationship between the widths of the chief absorption bands in 
the visible spectrum and the lengths of the absorption tube. The 
widths of the bands obtained with solutions of known composition 
were compared with those of unknown composition, and on the 

i W. F. Hillebrand and Q. E. F. Lundell, Applied Inorganic Analysis, p. 437 (John Wiley <fc Sons, New 
fork, N.Y., 1929). 

2 W. Prandtl and K. Scheiner, Z. anorg. allgem. Chem. 220, 107 (1934). 

3 B. Brauner, Chem. News 77, 161 (1898). 

* J. N. Friend and D. A. Hall, Analyst 65, 144 (1940). 

•Haas, Beitr. Kenntnis Pru. Nd. Dissert. Berlin 47, 51, 54 (1920). 

• G. Schumacher, Z. Kenntnis Samarium. Dissert. Berlin 14 (1921). 
7 L. F. Yntema, J. Am. Chem. Soc. 45,907 (1923). 

« E. Delauney, Compt. rend. 185, 354 (1927). 



Rodden] Praseodymium, Neodymium, and Samarium 559 

assumption that Beer's law applies, the concentrations of the salts 
giving bands of equal width were calculated from the inverse ratio 
of the lengths of the absorption tube. A photographic method of 
recording w r as used. 

The fourth method is based on the use of the spectrophotometer 
and was used initially by Muthmann and Stiitzel. 9 Praseodymium 
and neodymium in mixtures were determined by using a visual 
spectrophotometer. The band at 521 m/x was employed for neody- 
mium and the one at 480 m/i for praseodymium. The band at 480 
mju, however, is materially affected by neodymium and samarium, 
and therefore the results are somewhat in error. Partridge and 
Rodden 10 used filters of colored glass and gelatin to isolate certain 
bands corresponding to the absorption regions of neodymium, praseo- 
dymium, and samarium. A photoelectric means of recording was 
used. Though neodymium could be determined in mixtures of 
praseodymium and neodymium, the determinations of praseodymium 
and samarium in mixtures were not satisfactory. O. S. Planfinga n 
showed that neodymium could be determined in a mixture of praseo- 
dymium and neodymium by using a filter photometer w r ith a pho- 
tronic cell as the measuring device. In this procedure, 0.015 g of 
neodymium could be detected, and the deviation from the true value 
in most cases was of the order of 1 percent in equimolar mixtures 
containing up to 0.24 mole per liter of each ion. The measurements 
were affected by salts other than those of rare earths, and by the 
acidity. 

In preliminary experiments in this laboratory with a filter photom- 
eter, 12 an attempt was made to isolate certain narrow bands by 
using solutions of rare earth salts as well as glasses containing rare 
earth oxides. The results were unsatisfactory and indicated the 
necessity of using some type of spectrophotometer. In the work 
reported in the present paper a spectrophotometer 13 was used to 
determine spectral transmittancy curves of lanthanum, cerium, 
praseodymium, neodymium, samarium, europium, and gadolinium. 
Transmittancy-concentration curves for praseodymium, neodymium, 
and samarium were obtained. From these measurements it was 
possible to determine praseodymium, neodymium and samarium in 
mixtures of the three, together with lanthanum. Similar measure- 
ments have been made on the yttrium group of elements, and the 
results will be reported in a subsequent publication. 

II. EXPERIMENTAL 

1. APPARATUS 

A Coleman double-monochromator spectrophotometer, model 10 S 
was used, equipped with a slit stated by the manufacturer to select 
a spectral region of 5 mji. In this instrument the source of radiant 
energy is a line-coiled filament energized by a 4 -cell lead storage 

■ W. Muthmann and L. Stiitzel, Ber. deut. chem. Ges. 32, 2653 (1899). 

i" H. M. Partridge and 0. J. Rodden. Abstracts of the Indianapolis meeting of the American Chemical 
Society, (March 1931). 

u 0. S. Plantinga, A Study of the Colorimetric Determination of Neodymium. Dissertation, New York 
University (1934). 

ia B. A. Brice, Rev. Sci. Instr. 8, 279 (1937). 

is The cost of a spectrophotometer has, prior to the recent introduction of certain moderately priced photo- 
electric spectrophotometers, generally precluded its use in the analytical laboratory. 



560 Journal oj Research of the National Bureau of Standards [Voi.26 

battery of high capacity. The source remains substantially constant 
during the time required for the desired test. The radiant energy 
passes first through a diffraction grating attached to a condensing 
lens and next through a slit. The nearly homogeneous energy from 
that slit passes through a right-angled prism to which is attached a 
second diffraction grating which disperses the stray energy. A 
narrow exit slit then isolates the 5-m/z band of energy used in the 
measurements. This band can be selected anywhere in the range 
from 350 to 1,000 m/*. The homogeneous energy then passes through 
the absorption cell to a cesium oxide photocell. Absorption cells of 
two types were used: (1) Cylindrical cells of approximately 17 mm 
internal diameter which required 8 to 10 ml of solution; and (2) 
square cells with 13.08 mm between faces which required 5 to 6 ml of 
solution. B batteries supply the potential for the photocell. A dark 
current compensator is built into the spectrophotometer. A Cole- 
man electrometer, type 3E, was used in conjunction with the spectro- 
photometer to measure the transmittancy (T) u of the solution. 

The wavelength scale of the instrument was calibrated from 400 to 
750 mju, by means of rare earth glasses. 15 

2. MATERIALS 

Lanthanum. — The lanthanum oxide was of unknown origin. Its 
emission spectrum 16 showed arsenic, in the order of 0.1 percent, as 
w r ell as barium, calcium, and magnesium in the order of 0.01 percent. 
No other rare earth was detected. After ignition of the oxide at 
1,100° C, an amount equivalent to 2.5 g of lanthanum was dissolved 
in nitric acid, the solution was evaporated to dryness on the steam 
bath, and the residue was dissolved in water and diluted to 100 ml in 
a volumetric flask. Other solutions of lanthanum were prepared 
from this solution by diluting with water. 

Cerium. — The cerium nitrate used was prepared from the oxide 
obtained from one of the supply houses. The oxide contained 1.7 
percent of impurities, which were chiefly thorium, chromium, and 
aluminum. No rare earths were detected. The cerium oxide was 
converted to sulfate, precipitated with ammonia, and, after washing, 
was dissolved in nitric acid. Water was then added and the solution 
evaporated to dryness and made up to 0.0231 g of cerium per milliliter 
with water. 

Praseodymium. — The praseodymium oxide was obtained from the 
Charles James collection. Its emission spectrum showed faint traces 
of calcium, iron, magnesium, and silicon. The amount of lanthanum 
was estimated to be less than 1.0 percent and that of yttrium and 
cerium less than 0.01 percent each. No other rare earths were 
detected. The praseodymium oxide was ignited at 900° C, and an 
amount necessary to give 2.5 g of praseodymium, on the basis of 
PreOn/ 7 was dissolved in nitric acid and treated like the lanthanum 
preparation. 

14 The transmittancy is defined as the ratio of the transmission of a cell containing solution to that of an 
identical cell containing distilled water. 

" Acknowledgment is made to H. J. Keegan, of the Photometry and Colorimetry Section of this Bureau, 
for the transmission measurements of these glasses, made on the General Electric recording spectrophoto- 
meter. 

is Acknowledgment is made to B. F. Scribner and H. R. Mullin, of this Bureau, for emission-spectrum 
analyses of the salts used. 

" P. H. M-P. Brinton and H. A. Pagel, J. Am. Chem. Soc. 45, 1460 (1923). 



Rodden] 



Praseodymium , Neodymium, and Samarium 



561 



Neodymium. — The neodymium material was purified by the author 
by crystallization of the double magnesium nitrate. The emission 
spectrum of the oxide showed less than 0.01 percent each of calcium, 
magnesium, and silicon. No other rare earth was detected. A solu- 
tion containing 0.025 g of neodymium per milliliter was prepared in 
the way described under lanthanum. 

Samarium. — The samarium oxide was obtained from the Charles 
James collection and showed calcium, iron, magnesium, and silicon 
in amounts less than 0.01 percent each. Europium was present, but 
in amount less than 0.01 percent. A solution of 0.025 g of samarium 
per milliliter was prepared as described for lanthanum. 

Europium. — The europium oxide was obtained from the Charles 
James collection. Its emission spectrum showed the presence of 
gadolinium and samarium in the order of 0.1 percent each. A solu- 
tion containing 0.0182 g of europium per milliliter was prepared in the 
way described under lanthanum. 

Gadolinium. — The gadolinium oxide was obtained from the Charles 
James collection. Its emission spectrum showed the presence of 
europium as a major constituent. No other rare earths were detected. 
A solution containing 0.025 g of gadolinium (Gd+Eu) per milliliter 
was prepared in the way described under lanthanum. 

From these master solutions, mixtures which contained different 
amounts of the several elements were prepared. 

3. TRANSMITTANCY MEASUREMENTS 

Spectral transmittancy (ST) curves for lanthanum, cerium, praseo- 
dymium, neodymium, samarium, europium, and gadolinium nitrate 



100 


^ 


1 — 1 


> — 1 


>— N 


J 1 




, 


i 




, 


i 1 


w__ 


>— <> 






























z 
g 
85 G o 






















































a. 60 
>• 




























z 
< 




























i- 40 
I 




























h 20 






















































o 





























550 650 750 850 

WAVELENGTH- MILLIMICRONS 



950 



Figure 1. — Spectral-transmittancy curve of lanthanum nitrate solution containing 
0.25 g of lanthanum per 10 ml. 

Cylindrical cells employed. 

solutions are shown in figures 1, 2, 3, 4, 5, 6, and 7. For all but 
lanthanum, cerium, and gadolinium, readings were made at intervals 
of 10 mjit, except where intense absorption bands occurred, in which 
case 1- to 2-m/i steps were used in order to obtain the transmittancy 
and wavelength at minimum transmittancy. Readings obtained for 
lanthanum, cerium, and gadolinium were made at the wavelengths 
indicated in the figures. The ST curve for a mixture of praseody- 
mium, neodymium, and samarium is shown in figure 8. 

315285 — 11 7 



562 Journal of Research of the National Bureau of Standards [voi.se 



100 




























H 80 






















































s 

a 

* 60 

>- 

2 
n 

k 20 




























































































































































































450 550 650 750 850 
WAVELENGTH-MILLIMICRONS 



Figure 2. — Spectral-transmittancy curve of cerium nitrate solution containing 0.', 

g of cerium per 10 ml. 

Square cells employed. 



100 


rf 


1 




3XW- 


f 


r 


CPOQI 


XXCOC 


ooooc 


OCCgt 


oc^x 


OXV- 


"^ 


80 






J 


i 






















I 


( 
















CA 


























60 










IJ 


















40 








| 












































20 


















































































550 



450 550 650 750 850 

WAVELENGTH-MILLIMICRONS 



950 



Figure 3. — Spectral-transmittancy curve of praseodymium nitrate solution contain- 
ing 0.25 g of praseodymium per 10 ml. 

Cylindrical cells employed. 




350 



450 550 650 750 850 

WAVELENGTH-MILLIMICRONS 



Figure 4. — Spectral-transmittancy curve of neodymium nitrate solution containing 
0.25 g of neodymium per 10 ml. 

Cylindrical cells employed. 



Hodden] 



Praseodymium, Neodymium, and Samarium 



563 



80 



20 



rJl 


i 


f 


^y>>Ol^^y)000000<X>OOOOOOOOOOoCK 


XJOOCK 


>»Vf 


1 


r 


















1 









































































































































































































450 



Figure 5. 



550 650 750 850 
WAVELENGTH- MILLIMICRONS 



Spectral-transmittancy curve of samarium nitrate solution containing 
0.25 g of samarium per 10 ml. 

Cylindrical cells employed. 



z 80 



rfi 


f 


xxyycc 


<?TT_t 


,-«>•>■< 


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txoa 


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1 


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350 



550 650 750 850 

WAVELENGTH-MILLIMICRONS 



950 



Figure 6. — Spectral-transmittancy curve of europium nitrate solution containing 
0.182 g of europium per 10 ml. 

Square cells employed. 



80 



40 



O— — O— — O— — O 



550 



450 550 650 750 850 

WAVELENGTH-MILLIMICRONS 



Figure 7. — Spectral-transmittancy curve of gadolinium nitrate solution containing 
0.25 g of gadolinium per 10 ml. 

Square cells employed. 



564 Journal of Research oj the National Bureau oj Standards [Vol. m 

The only band which can be used for the determination of praseo- 
dymium in mixtures is the one with minimum transmittancy at 446 



A 






1/ 




r 


'V 


1 


A 


f 


Ill 


Ajto 


A 




v 


I 












y 






t 














1 














, 






I 






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c 


















' 






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! 

















































































































350 450 



550 650 750 850 

W AVELENGTH- MILLIMICRONS 



Figure 8. — Spectral-transmittancy curve of a nitrate solution containing 0.050 g of 
neodymium, 0.075 g of praseodymium, and 0.075 g of samarium per 10 ml. 

Cylindrical cells employed. 

m/i, as neodymium and samarium interfere with the use of the band at 
469 m/i, and neodymium with the one at 590 m/*. As can be seen in 
figure 5, the band at 446 m/x is affected somewhat by samarium. 



100 



80 



60 



40 



20 



V 


























^ 


V 


























*N 


























V 


\ 
























N 


b 


s 


























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^ 


^ 

































04 



.08 .12 .16 .20 

CONCENTRATION- GRAMS PER 10 ML 



•24 



Figure 9. 



-Transmittancy-concentration curves of praseodymium nitrate solutions 

at 446 mp. 

A, Values obtained using square cells. 
Ot Values obtained using cylindrical cells. 

The bands of neodymium nitrate at 521, 742, 798, and 870 ma are, 
fortunately, free from interference by praseodymium or samarium. 
These bands differ considerably in amount of absorption, the one at 
798 m/z absorbing the most strongly and therefore yielding the most 
sensitive test procedure. 



Hodden] 



Praseodymium, Neody7niurn i and Samarium 



565 



The absorption band of samarium nitrate at 402 m/i is the only one 
which can be used for this element, because the absorption at 949 m/* 
is rather low and praseodymium and neodymium interfere at 479 m/z. 
The bands of samarium show less absorption than the other elements 
of the cerium group except europium, hence, samarium can be deter- 
mined less precisely than neodymium and praseodymium. 

Transmittancy-concentration curves for praseodymium, neodym- 
ium, and samarium were obtained with both square and cylindrical 
cells. 

The transmittancies of praseodymium nitrate solutions were 
obtained at 402, 446, and 521 m/x, at concentrations ranging from 
0.0125 to 0.25 g of praseodymium per 10 ml. (This material may have 
as high as 1 percent of lanthanum.) The results for the band at 446 



100 




.08 .12 .16 .20 

CONCENTRATION-GRAMS PER 10 ML 

Figure 10. — Transmittancy-concentration curves of neodymium nitrate solutions a 

the wavelengths indicated. 

A, Values obtained using square cells at 521 mp. 
O, Values obtained using cylindrical cells at 521 mju. 
D , Values obtained using cylindrical cells at 578 m/i. 
#, Values obtained using cylindrical cells at 798 mp. 

m/j are shown in figure 9. The deviation from Beer's law in this 
case is evident. The results at 402 m^ are shown in figure 12 on an 
enlarged linear scale. The effect of praseodymium at 521 my is also 
indicated in this figure. 

Transmittancy-concentration curves for amounts of neodymium 
ranging from 0.0025 to 0.25 g per 10 ml are shown in figure 10 for the 
521, 578, and 798 m/i bands of neodymium nitrate. Beer's law holds 
closely for the 521, 578, and 798 m^t bands in concentrations up to 
0.10 g per 10 ml. There is a slight deviation from Beer's law for the 
band at 521 m/i when the concentration increases to 0.25 g per 10 ml. 
The values at 402 and 446 m/x are shown on an enlarged linear scale 
in figure 12. 

The transmittancy-concentration curves of samarium nitrate for 
the band at 402 m/i, for concentrations ranging from 0.025 to 0.25 g of 






566 Journal of Research oj the National Bureau oj Standards [Voi.se 

samarium per 10 ml, is shown in figure 11. The transmittancies at 
446 and 521 m/i are shown in figure 12. 



100 
80 

60 

i 

I 40 



2 
h 20 



10 



*.JK_„ 



04 



.24 



.08 .12 .16 .20 

CONCENTRATION-GRAMS PER 10 ML 

Figure 11. — Transmittancy-concentration curves of samarium nitrate solution at 

402 m\i. 

A, Values obtained using square cells. 
Oi Values obtained using cylindrical cells. 

The transmittancy-concentration curves show that the amount of 
each rare earth element in a solution for analysis should not exceed 

100 
99 

98 

100 
99 

»_ 100 
§ 99 

QE 

u 98 

I 99 

5 98 

5 97 

DC 

»" 96 

95 

100 

99 



too 













Pr- 402 






















j— ' 




JU 






















1" 




Nd- 402 


1 1 


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Sm-921 


1 1 1 1 1 1 1 1 1 1 i 1' 



.04 .08 .12 .16 .20 

CONCENTRATION-GRAMS PER 10 ML 



Figure 12. — Transmittancy-concentration curves of samarium, neodymium, and 
praseodymium nitrate solutions at the wavelengths indicated, using square cells. 

approximately 0.20 g per 10 ml. This is about the maximum which 
can effectively be used, especially in the case of samarium, concentra- 



Hodden] 



Praseodymium, Neodymium, and Samarium 



567 



tions of which greater than 0.20 g per 10 ml in the square cells change 
the transmittancy but slightly. 

It will be noted from figure 12 that the presence of other rare earths 
will affect the transmittancy measurement of each element in question 
to a certain extent, and that the 446 m/x band, which is used for the 
praseodymium determination, may be thus affected to the greatest 
extent. It is possible, however, to correct for the effect of these differ- 
ent elements, as will be shown later. The effect of lanthanum on 
small amounts of other rare earths in the cerium group was studied to 
see if any noticeable wavelength shift in bands was observed, as was 
reported by Quill, Selwood, and Hopkins. 18 In the concentrations 
used, the shift of the bands, if present, was not sufficient to affect the 
results. Figure 8 shows that no noticeable shift in the wavelengths of 
minimum transmittancy of the bands was observed in a mixture of 
praseodymium, neodymium, and samarium. 

III. RESULTS OBTAINED 

A tabulation of the data obtained with a mixture of 0.075 g of 
samarium, 0.075 g of praseodymium, and 0.050 g of neodymium in a 
volume of 10 ml in square cclis is shown in table 1. 

Table 1. — Illustration of the data obtained in the analysis of a solution of samarium t 
praseodymium, and neodymium 



1 


2 


3 


4 


5 


6 


7 


8 


Element 


Wave- 
length 


Apparent 

transmit- 
tancy 


Transmit- 
tancy of Pr 

at wavelength 
of col. 2, 

excluding 446 


Transmit- 
tancy of Nd 
at wavelength 

of col. 2, 
excluding 521 


Transmit- 
tancy of Sm 
at wavelength 

of col. 2, 
excluding 402 


Corrected 
transmit- 
tancy of 
element listed 
in col. 1. Col. 
3-S-4X5X6 


Amount of 
element 


Present 


Found 


Sm_. 


my. 
402 
446 
521 


Percent 
82.4 
53.6 
80.4 


Percent 
99.7 


Percent 
99.8 
99.6 


Percent 


Percent 

82.8 
54.7 
80.5 


Q 
0. 075 
.075 
.050 


0. 075 


Pr 


98.5 
100.0 


.076 


Nd 


99.9 


.052 









The transmittancy of the solution at 402, 446, and 521 m/i is 
entered under column 3. From the transmittancy-concentration 
curves 19 of samarium, praseodymium, and neodymium, as given in 
figures 11, 9, and 10, it is noted that the apparent transmittancy given 
in column 3 corresponds to approximately 0.079 g of samarium, 0.080 
g of praseodymium, and 0.054 g of neodymium per 10 ml. From 
figure 12, which shows the transmittancy due to samarium, prase- 
odymium, and neodyirium at the various wavelengths, are obtained 
the values which are entered in columns 4, 5, and 6. The transmit- 
tancy due to the element in question is then found by dividing the 
values in column 3 by the product of the values in columns 4, 5, and 6. 
This value is entered in column 7. From the transmittancy-concen- 
tration curves of figures 9, 10, and 11, the concentrations of the various 
elements are found and listed in column 8. 

is L. L. Quill, P. W. Selwood, and B. S. Hopkins, J. Am. Chem. Soc. 50, 2929 (1928). 
*• Since the transmittancy curves may vary importantly with slit-width and cell thickness, analysts should 
not use the data in this paper hut should obtain their own standard curves. 



568 Journal of Research oj the National Bureau of Standards Woi. to 

The data in table 2 illustrate the results obtained by the method 
when applied to mixtures of praseodymium, neodymium, and 
samarium. 



Table 2. — Results obtained in the analysis of nitrate solutions containing varying 
amounts of Pr, Nd, and Sm 



Mixture 


Concentration 


Amount 
present 


Amount found 


Cylindrical cells 


Square cells 


Samarium 


g/10 ml 
0.025 
.200 

.200 
.025 

.150 
.025 

.025 
.150 

.050 
.075 
.075 

.200 
.005 

.200 
.010 

.200 
.005 


g/10 ml 

0.025 
.202 

.200 
.026 

.160 
.025 

.028 
.151 

.051 
.075 
.074 

Not determined 
.005 

Not determined 
.010 

Not determined 
.006 


g/10 ml 

0.025 

.198 

.199 
.026 

.150 
.026 

.030 
.153 

.052 
.076 
.075 


Neodymium 


Samarium 


Neodymium 


Neodymium 


Praseodymium 


Neodymium _ 


Praseodymium 


Neodymium _ 


Praseodymium 


Samarium 


Lanthanum.. 


Praseodymium 


.005 


Lanthanum 


Samarium. 


.012 


Lanthanum. __ 


Neodymium.. 


.006 





IV. DISCUSSION 

Lanthanum, cerium, and gadolinium all show the same type of 
general "absorption" (1-T) from 350 to 450 mju. Extremely fine 
particles of dust, filter-paper fibers, and the like, which scatter the 
light at short wavelengths, may be the cause of this effect. It is 
recommended that all solutions used be centrifuged to remove as 
many particles as possible before transmittancy measurements are 
made. 

The variation in reading transmittancy is seldom greater than 
±0.2 unit on a scale of 100.0 units. The reproducibility of the wave- 
length setting is illustrated by the following data obtained for a solu- 
tion of neodymium. Each reading represents a separate setting of 
the wavelength scale. 



* 



Wavelength 
setting 


Percent transmittancy 


mix 
729 
730 
731 


76.8 
69.5 
62.7 


76.7 
69.5 
62.9 


76.6 
69.4 
62.9 



Hodden] Praseodymium, Neodymium, and Samarium 569 

The data in table 2 show that deviations greater than ±3 mg are 
usually not obtained. 

The presence of 2 mg of praseodymium can be detected when the 
cylindrical cells are used and 1 mg with the square cells. The sensi- 
tivity of the neodymium test depends on the band chosen; 0.5 mg in 
6 ml can be detected at 798 m/*. When the 521 m/x band is used, 1.5 
mg of neodymium can be detected. Samarium does not give a very 
sensitive test, 3 mg of it in a volume of 6 ml being the smallest amount 
of this element which can be detected. 

Europium, whose nitrate ST curve is shown in figure 6, seriously 
affects the determination of samarium. However, the amount of 
europium can be determined by reduction and subsequent oxida- 
tion. 20 The transmittancy indicated for samarium could then be 
corrected for the interference of europium by means of a transmit- 
tancy-concentration curve of europium. Gadolinium, whose nitrate 
ST curve is shown in figure 7, has no bands from 350 to 1,000 niju, but 
has a slight absorption similar to that of lanthanum near 350 mju. 
The ST curve of cerium nitrate is shown in figure 2 to indicate the 
effect of this element. 

The applications of this spectrophotometry method in rare earth 
chemistry are many. The use of certain rare earths in the ceramic 
industry has introduced the problem of determining neodymium. 
As such mixtures are usually confined to members of the cerium 
group, a determination of neodymium can be readily made. The 
application of the method in following fractional crystallizations 
during the separation of rare earths is noteworthy. Impurities may 
be determined in so-called pure rare earth salts. 

In a mineral analysis, the group consisting of lanthanum, cerium 
praseodymium, neodymium, samarium, europium, and gadolinium 
can be separated from the other group, but, as stated in the introduc- 
tion, the separation is not sharp. However, by adding sodium sulfate 
until the neodymium absorption bands disappear it will be found that 
practically all of the cerium group is removed, contaminated some- 
what by small amounts of some of the elements of the yttrium group. 
In the usual method of mineral analysis the cerium group is separated 
from the yttrium group, and cerium then determined by oxidation 
and titration. 21 Cerium is reported as Ce 2 3 and the remainder of 
the cerium group as mixed oxides of the form R 2 3 . The total sum- 
mation of the analysis of the mineral is then somewhat in error, as 
praseodymium forms a higher oxide whose composition in mixtures 
is unknown (see footnote 17). 

The following procedure is suggested for the analysis of the cerium 
group. The rare earths are separated first from other elements by 
some standard procedure. The cerium group is then separated from 
the yttrium group as the sodium double sulfates. Cerium is sepa- 
rated from the other elements of the cerium group 22 and determined 
by oxidation and titration. As the lanthanum is determined by 
difference, it is necessary to treat the rare earth oxide mixture with 
hydrogen at 900° C to convert the black oxide of praseodymium to 

20 H. N. McCoy, J. Am. Chem. Soc. 58, 1577 (1936). 

« H. H. Willard and P. Young, J. Am. Chem. Soc. 50, 1379 (1928). 

22 The author has found that this can he done quite satisfactorily by precipitating cerium as cejic hydroxide 
with zinc oxide. The bulk of the zinc is then separated from the rare earths by precipitating the rare earth 
hydroxides with a large excess of ammonia. The small amount of zinc which remains with the rare earths 
is removed by means of hydrogen sulfide in 0.01 iV sulfuric acid. The rare earths are then precipitated with 
oxalic acid and the oxalates ignited to the oxides. This procedure will be published in a subsequent paper. 






570 Journal of Research of the National Bureau of Standards [Voi.s6 

Pr 2 3 . The mixed oxides are then weighed. (Cerium is removed 
before the treatment with hydrogen, since it is very difficult to reduce 
all of the Ce0 2 to Ce 2 3 .) If necessary, europium is determined by 
the method of McCoy (see footnote 20), but it occurs in such small 
amounts in minerals that its presence can ordinarily be neglected. 
The mixed oxides, consisting chiefly of lanthanum, praseodymium, 
neodymium, and samarium, are converted to nitrates by dissolving 
in nitric acid and evaporating to dryness on a steam bath. The 
nitrates are dissolved in water and diluted to a known volume. The 
solution is now centrifuged to remove dust particles. Praseodymium, 
neodymium, and samarium are then determined by measuring their 
transmittancies at the proper wavelengths. The equivalent amounts 
of the several rare earths are calculated, as described in section III, 
from transmittancy-concentration curves constructed from trans- 
mittancy measurements on solutions prepared from salts of high purity. 
Lanthanum is obtained by difference. Gadolinium has no absorption 
in the visible region of the spectrum and will be included in the 
lanthanum value. 

Washington, November 20, 1940.